The hydrocracking technology has features of high adaptability of raw material, high flexibility of product scheme, high selectivity of target product, high product quality, high additional value and the like, and the hydrocracking technology can directly transform various heavy inferior raw materials into clean fuel oil and high quality chemical raw materials. The hydrocracking technology has become one of the most important heavy oil deep conversion processes in modern refining and petrochemical industry and is widely used in the world. The hydrocracking technology has become the core technology of “refining-chemical-chemical fiber” combining in modern refining industry. The hydro-upgrading technology transforms various inferior diesel oil into high quality diesel oil or mixed components thereof in a moderate process condition, enables efficiently improving quality of the diesel oil, especially significantly reduces density of the diesel oil, aromatic content, S and N content and T95 point, and significantly increases the cetane number of the diesel oil. Some catalyst further has the capability of reducing the solidification point of the diesel oil. Thus the hydro-upgrading technology is ideal technology for upgrading inferior diesel oil for refining enterprise.
The current key components for cracking in hydrocracking catalyst are usually Y zeolite and β zeolite. Compared with Y zeolite, β zeolite has three dimensional 12-membered ring aperture structures instead of a supercage structure in Y zeolite, and has a main feature of bi-6-membered ring unit cavity structure with two 4-membered rings and four 5-membered rings, which belongs to cubic system. The silica alumina structure of the β zeolite has diversity and complexity. The skeleton structure of the β zeolite is more complex than that of the Y zeolite. Two linear pores in three mutually crossed pore systems are mutually orthogonal and vertical to [001] direction, in which the pore has a size of 0.57 nm×0.75 nm. The third 12-membered ring pore system is parallel to the [001] direction and is a non-linear pore, in which the pore has a size of 0.56 nm×0.65 nm. The skeleton silica alumina structure of the fully crystallized β zeolite also has diversity. The skeleton silica alumina structure is a four-coordinated structure and is the main body of the total silica alumina existence form in the zeolite, and its basic structure consists of Si(4Al), Si(3Al), Si(2Al), Si(1Al), and Si(0Al) structural units of different contents and is mainly in the form of Si(3Al) and Si(2Al). In addition, the zeolite further has six-coordinated non-skeleton aluminum. The silica alumina existence form and content of these structures vary in subsequently different modifying processes, thereby causing different catalytic performances.
The existing method for modifying a β zeolite (e.g., CN1105646A) generally comprises first ion exchanging with ammonium to remove sodium, calcining at a high temperature to remove the occluded template (organic amine), and conducting dealuminization and hydrothermal treatment at a constant pressure, to significantly increase the silica to alumina ratio of the β zeolite. Particularly, in the process of calcining at a high temperature to remove amine, patents such as CN1157258C, CN1166560C, etc. focus on segmented calcination to remove amine, which introduce complex preparation process. Moreover, before segmented calcination to remove amine, the zeolite is subjected to ammonium salt solution to replace sodium. Sodium ion here is a balance of the negative charge in the zeolite skeleton (generally formed by the skeleton aluminum), whereas the treatment for removing organic amine by calcination after sodium removal (regardless of one-step high temperature treatment or multi-step treatments) intensifies the skeleton dealuminization of the zeolite with non-selective skeleton dealuminization, which causes an inhomogeneous skeleton structure of the modified zeolite and large defects, and forms a large number of six-coordinated non-skeleton aluminum structure in the pore (which blocks the pore, partially covers the acid center of the skeleton and easily causes undesired cracking reaction). The subsequent acid treatment or hydrothermal treatment continues destroying the skeleton structure of the zeolite, such that there exist Si(X—Al) structures with different proportions and a certain amount of non-skeleton structures in the zeolite, and the zeolite has acid centers with different strengths showing different cracking performances, which can significantly affect the selectivity of the target product. Precisely because of the complexity of the silica alumina structure in the β zeolite, using the abovementioned modifying methods causes inhomogeneous skeleton structure of the modified zeolite and directly affects the acid strength and acid density of the modified zeolite, thereby affecting the performance of the hydrocracking catalyst.
CN101450318A discloses a method for modifying a β zeolite. The method comprises exchanging the sodium type β zeolite with ammonium salt, and then impregnating the zeolite with a phosphorous compound solution and a transition metal compound solution, to yield a β zeolite with larger BET surface area and higher relative crystallinity, which can be further subjected to shape-selective cracking to generate micromolecule olefin.
CN1393522A discloses a method for modifying a β zeolite. The method comprises the following processes: (1) directly conducting ammonium salt exchanging on the fully crystallized β zeolite, (2) filtering, washing, drying and calcining the β zeolite after ammonium salt exchanging, (3) conducting an acid treatment on the β zeolite after calcining to remove ammonium, and filtering, (4) conducting hydrothermal treatment under pressure on the β zeolite after the acid treatment. In the method, the β zeolite is firstly subjected to an inorganic acid treatment, and then to a hydrothermal treatment, the skeleton structure of the zeolite is partially destroyed in such a process, the crystallinity of the zeolite is decreased, and a large bulk of non-skeleton structure left in the pore of the zeolite which is difficult to remove, and the acid distribution and acid strength of the modified zeolite will be affected. In addition, the method further comprises a high temperature hydrothermal treatment after the acid treatment, forming a certain amount of non-skeleton aluminum in the zeolite, which directly affects the pore structure and acid property of the modified zeolite. The change of the acid distribution and acid property of the zeolite will directly affect performance of the catalyst prepared by taking the modified zeolite as the cracking component, and especially on the property of the diesel oil and chemical materials. Moreover, the process of modifying the zeolite in this method takes a long time, and the yield of the target zeolite in the preparation process is low. Meanwhile, the multi-step modification treatment greatly increases the modification cost and energy consumption. U.S. Pat. No. 5,350,501, U.S. Pat. No. 5,447,623, U.S. Pat. No. 5,279,726, and U.S. Pat. No. 5,536,687 introduce a catalyst containing both a β zeolite and a Y zeolite. The catalyst comprises the following components when used for producing middle distillate oil: a Y zeolite (1-15 wt %), a β zeolite (1-15 wt %), amorphous silica alumina, alumina, and metal W and Ni. The β zeolite used therein is a hydrogen β zeolite obtained by ion exchanging and calcining to remove the template. The catalyst does not have high activity and selectivity of middle distillate oil, which is difficult to meet the need of improving the throughput of the device and further increasing production of the middle distillate oil for the refinery.
CN1393521A discloses a middle distillate oil hydrocracking catalyst and preparation method thereof, wherein the carrier of the catalyst is amorphous silica-alumina, alumina, and a complex zeolite of Y and β. The complex zeolite is obtained by calcining the β zeolite raw powder at a high temperature to remove the template, mixing with modified Y zeolite, and then treating with a mixed solution of H+ and NH4+. In this method, first calcining the β zeolite raw powder at a high temperature to remove the template will affect the skeleton structure of the zeolite, significantly decrease the crystallinity of the zeolite and affect the acidity. The catalyst prepared in this way does not have high activity, and the product quality of the middle distillate oil of the aviation kerosene and diesel oil still needs to be further improved.